If Google wanted a second language on Android, they'd have it by now. Either golang or JS+V8 (possibly out-of-browser) would have been fine choices. As well, Dart's stated goal is "structured web programming", and parts of it's design are constrained by the need to run within the browser and compile down to JS. Imposing those same restrictions on Android development would be counterproductive.
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If Google wanted a second language on Android, they'd have it by now. Either golang or JS+V8 (possibly out-of-browser) would have been fine choices. As well, Dart's stated goal is "structured web programming", and parts of it's design are constrained by the need to run within the browser and compile down to JS. Imposing those same restrictions on Android development would be counterproductive.